Light Emitting Diode (LED) strings are used in a variety of applications. For example, LED strings may be used for applications including, but not limited to, RGB backlighting for flat-panel displays, signage, decorative lighting, and automotive markets.
Referring to FIG. 1, the current architecture for driving arrays of LED strings is shown. The goal of an LED driver is to process the input line voltage, VIN, into the proper DC current for driving the LEDs. The heart of an LED driver is a pulse-width modulation (PWM) controller that regulates and controls the current flowing through the LEDs during operation. A key requirement of the PWM circuit is to drive the LEDs at a constant current to provide uniform and consistent illumination.
As part of the conventional method for driving arrays of LEDs, the PWM controller embodies several circuit blocks, such as a clock generator, voltage reference circuit, and PWM controller circuit with external PWM dimming input (PWM_D). FIG. 2 illustrates a typical PWM peak current-mode control architecture. The current sense input provides a feedback voltage proportional to the current flowing through the load (i.e. LED). A comparator (CM) block determines if this voltage is higher or lower than reference voltage VREF, and switches the gate output appropriately to maintain a constant current flowing through the load. An external low-frequency PWM dimming (PWM_D) input allows dimming of the LED light output by taking control of the gate output. It should be noted that such a controller is not specific to any one type of power supply topology.
The problem with the current architecture is that it requires redundant circuitry to control the LED strings. The redundant circuitry causes an increase in die size and cost. Furthermore, in current architecture, the LEDs are not synchronized to a common clock, resulting in non-uniform illumination, beat frequencies, and EMI effects.
Therefore, it would be desirable to provide a system and method to overcome the above problems.